136 LIFE AND EXPERIENCES CHAP. 



Has electric or other light also been used in the application 

 of the principle of spectrum analysis to mineral waters? or 

 their contents ? 



In the lecture which I gave at the Royal Institution 

 on the " Opalescence of the Atmosphere " on June i, 

 1866, I explained the action of finely divided 

 matter in absorbing and scattering different rays of 

 light, occasioning red sunsets, and the orange-coloured 

 sun seen through a fog, &c. I wished on this 

 occasion to illustrate my subject by Faraday's 

 researches on the colour of gold films, in which he 

 showed that finely divided gold imparts magnificent 

 purple and green colours, as seen by transmitted light, 

 to the medium, whether liquid or solid, through which it 

 is diffused. I found in the laboratory of the Institution 

 the bottles containing the finely divided gold sus- 

 pended in water, in which, though the vessels had 

 been standing for years, the heavy gold had not been 

 deposited. I did not, however, find in the laboratory 

 any of the gold films which he had prepared with 

 gelatine, and I went up into his rooms at the top of the 

 Institution, to ask him if he would be good enough to 

 lend me some of these which he had preserved in 

 watch-glasses, and put away in a box. 



His mind was then failing, and it was quite sad to 

 see that he hardly understood what I was asking for. 

 Mrs. Faraday, who stood close by, tried to recall the 

 facts to his mind and said to him : " Don't you 

 remember those beautiful gold experiments that you 

 made ? " To which he replied in a feeble voice : 

 "Oh, yes, beautiful gold, beautiful gold," and that was 

 all he would say. Mrs. Faraday found the box, and I 

 showed the specimens in the evening, and projected 

 the image of the coloured films on the screen. 



After the lecture was over I went to an evening 



